1. Field of the Invention
This invention relates generally to operational amplifiers and, more particularly, to an operational amplifier second stage into which a system zero has been introduced to help cancel a system pole and thus broadband the amplifier.
2. Description of the Prior Art
In most feedback operational amplifiers currently in use, an input gain stage drives a second stage consisting of at least one emitter follower transistor (or perhaps the input transistor of an emitter follower Darlington pair) which in turn drives a second stage gain transistor. The output of the circuit is generally coupled to the collector of this gain transistor by emitter follower output transistors. A capacitor (Miller) is usually coupled between the collector of the second stage gain transistor and the base of the first emitter follower transistor (i.e. the output of the input stage).
Since negative feedback is generally employed in an operational amplifier, a DC phase shift of 180 degrees is usually inherent in the circuit from input to output. It would therefore be desirable not to add additional negative phase shift into the circuit at higher frequencies to assure that the total phase shift from input to output does not exceed 360 degrees when the open loop amplifier is exhibiting voltage gain to ensure unity gain closed loop stability. However, the typical circuit described above purposely provides a dominant pole (a 6db/octave frequency dependent amplitude decay) which results in the introduction of an additional negative phase shift of 90 degrees in the limit at high frequencies. Although the total phase shift for the amplifier at frequencies above the pole frequency is now 270 degrees, the amplifier's open loop gain is reduced by 6db/octave as a result of the dominant pole. Thus, the pole is strategically placed to assure that at high frequencies, the amplifier voltage gain is reduced to less than one before an additional 90 degrees of excess negative phase shift is accumulated due to other parasitic poles within the amplifier (bringing the total negative phase shift to 360 degrees.) It should be clear that if the first parasitic pole is either cancelled or removed, the dominant pole can be placed higher in frequency to achieve a higher amplifier unity gain stable bandwidth.